A technique for isotope separation, specifically enrichment of the uranium U.sub.235 isotope, is shown in copending application Ser. No. 25,605, filed Mar. 25, 1970 and in corresponding French patent 71.14007, Jan. 10, 1971, both incorporated herein by reference. The system therein disclosed operates by generating a vapor of uranium metal which expands as a predetermined particle flow. The U.sub.235 isotope is selectively ionized by application of a narrow bandwidth, precisely tuned laser radiation to selectively excite and ionize only the U.sub.235 isotope without substantial ionization of the other isotopes. Once the particles of the desired U.sub.235 isotope have been ionized, they are accelerated toward collection surfaces and out of the general particle flow by, typically, crossed-field magnetohydrodynamic forces.
The efficiency and effectiveness of the system depends upon gathering at the collection surfaces a maximum of U.sub.235 particles and a minimum of all other components in the vapor flow. By using a narrow bandwidth, suitably tuned laser radiation for the selective ionization, a very great increase in the proportion of ionized U.sub.235 particles as contrasted to laser ionized particles of other isotope types can be achieved. Nevertheless, background ions existing in the particle flow prior to laser ionization will typically reflect the normal distribution between U.sub.235 and other uranium particles and will accordingly be composed primarily of U.sub.238 particles. These particles will, of course, be accelerated by the same MHD forces which accelerate the selectively ionized particles and will accumulate on the collection surfaces along with the U.sub.235 particles to reduce the efficiency of the separation process.
Where elemental uranium is employed as a source from which the particle flow is generated, substantial energy must be imparted to the solid uranium to raise it to the vapor state. The presence of this and other energy will operate through phenomena such as particle collisions, or thermal distributions to create a substantial number of ionized particles of uranium in the vapor flow prior to the application of ionizing laser radiation. As a result, the loss in separation efficiency through collection of the background ions can be significant. BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a system is disclosed for distinguishing between background ions and selectively ionized particles for specific isotope separation and collection.
In the preferred embodiment of the present invention for uranium enrichment, a uranium vapor flow is created by vaporizing from the surface of a mass of elemental uranium metal. The radially expanding vapor flow is sequentially pulsed with a magneto-hydrodynamic force to accelerate the background charged particles in a first direction, then selectively ionized by applied laser radiation to excite particles of the desired isotope type into the ion state and finally pulsed with a further magnetohydrodynamic force to decelerate the accelerated background ions and to accelerate the selectively ionized particles of the desised iostope type onto trajectories for impact on collection plates. These plates are typically oriented so as not to intercept the general particle flow but only the deflected particles of the desired isotope type.
As a result of the process and apparatus of the present invention, a substantial improvement is realized in the efficiency of the enrichment system. In addition, the particles which accumulate on the collection plates, being of a higher purity in the desired isotope, tend to accumulate at a slower rate and, therefore, the plates do not require cleaning and replacement as frequently. In addition, the system of the present invention permits the generation of electrical current in regions other than where ionizing radiation is applied without having an average non-zero component that might disturb the charged particle distribution responsible for providing conductivity in the MHD system.